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Anonymous recursion in PHP

The scenario

You’re neck deep in 3-year old production code, amazon one-click checkout isn’t working, and your wife just left you for someone who “doesn’t spend so much time writing unit tests.” You need to save the world by implementing an algorithm using an anonymous function now. Wait, what if the problem you need to solve is well served with a recursive approach? Maybe you attack the problem optimistically, starting with something like this:

Well, crap,” you say, seeing the Fatal error reported by PHP. Turns out that variables called as functions must be an instance of Closure, an instance of a class which implements __invoke(), or a string representing a named function in the global namespace. In the anonymous function body above,$fibonacci is none of these. It is an undeclared, free variable in the closure created by the anonymous function. At the time when it’s called, it hasn’t been bound—hence the Notice that you would have gotten if error reporting were set at a high enough threshold—and therefore can’t be called asanything, let alone as a function.

So how should you proceed? The classical solution would be simply to name this function; instead of binding the function to a variable called$fibonacci, we should just name the function fibonacci. That would certainly be convenient. But what if we encounter a higher-order function that expects a function as a parameter? It happens that many higher-order functions in PHP can simply accept as callbacks the names of functions, but I prefer not to rely on that. In fact, any userland function you write which expects a lambda will work just as happily being sent a string. But what if wemust provide the higher-order function with the means of calculating the nth Fibonacci number? How could we even go about implementing the recursion required to pull it off? I’m so glad you asked.

Naming the nameless

The difficult part about the above is that we have no good way to actually call the function in whose body we are executing while we’re still in it. The thing’s anonymous, after all—we are essentially looking for some way to break its anonymity, to call the nameless by name. Well, we may not be able to actually extract the name of this function for the purpose of calling it again, but we do have some information about the location where it’s held in memory. This location is known to $fibonacci, the value we’re using in the parent scope to hold the function itself. Therefore, if we could grab the value of $fibonacci, we could actually use it to recurse. The problem here is that $fibonacci does not exist in the scope where we need to use it.

If you had read my article about closures, you would know that we have the ability in PHP to do something that sounds very much like this. With PHP’suse keyword for anonymous function definition, we can pull either a value or a reference into a function’s scope. Let’s be naive and pull in the value of$fibonacci in a forshadowingly misguided attempt to recurse: